Dogs can tell how other dogs are feeling from the way their tails are wagging(摇摆)，according to researchers who monitored the animals' heart rate as they watched dogs' movies. The Italian team found that dogs had higher heart rates and became more anxious when they saw others wag their tails more to the left，but not when they wagged more to the right, or failed to wag at all.
The curious form of communication is probably not intentional, or consciously understood, but is instead an automatic behavior that arises from the structure of the brain, said Giorgio. “It's not something they clearly and exactly understand,” Giorgio told The Guardian. “It's just something that happens to them.”
Giorgio traces the effect back to the way the two halves of the brain process different experiences. In a previous study, his team showed that when a dog had a positive experience, activity rose in the left side of the brain, bringing about more tail wagging to the right. Or else more tail wagging to the left. The effect is barely visible to the human eye because dogs tend to wag their tails too fast, but it can be seen with slow motion video, or in some larger types.
In the latest study, the researchers wanted to find out whether the direction of tail wagging had any effect on other dogs. To get an answer, they fitted dogs with vests that recorded their heart rates, and played them movies of other dogs wagging their tails one way and then the other. To ensure the dogs reacted only to tail wagging, and not appearance? They repeated the experiment with dogs that appeared only as shadows.
“When dogs saw other dogs wagging their tails to the right, there was quite a relaxed reaction and no evidence of an increased heart rate. But when the wagging was to the left we saw an increase in heart rate and a series of behaviors typically associated with stress, anxiety and being more watchful, “Giorgio said. The anxious animals held their ears up, breathed, and kept their eyes wide open. The study appears in the latest issue of Current Biology.
